Britain has already said it will leave the customs union - the EU's tariff-free trading area - after Brexit, but Unite is demanding a U-turn.

Speaking to the Derby Telegraph, Simon Hemmings - Unite's chief staff negotiator for Rolls-Royce in Derby – said the Government’s stance will have a devastating impact on jobs at Rolls-Royce’s civil aerospace campus in Sinfin.

This is despite Rolls-Royce announcing a £150m upgrade of its East Midlands facilities – including a new £90m test bed in Derby – as recently as June.

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Mr Hemmings said: “If we’re not in the customs union, jobs will be lost. [The £150m announcement in June] protected this generation of engines and built the foundations for the next generation. If we get a hard Brexit, those foundations will not be built on.

“The production of the next generation of engines will not be built in the UK. They will be built in Germany or America, because of the costs associated with moving stuff across the Channel. The company will not want to incur those costs by continuing to assemble engines in the UK.”

Asked what that would mean for Rolls-Royce’s aerospace campus at Sinfin, Mr Hemmings said: “It would mean a slow decline. For aerospace, we've got five years guaranteed [with no compulsory job losses as a result of the £150m investment]. After that, it would start to decline very slowly.”

Mahf Khan (left) and Simmon Hemmings from Unite fear a hard Brexit will result in Rolls-Royce shredding jobs in Derby

Mr Hemmings said the £90m test bed announced for Derby would continue to be used for “experimental” work, but that new engines would start to be assembled in Germany or America.

“The critical issues for Rolls-Royce from a UK perspective are customs union and membership of the European Aviation Safety Agency (EASA), both of which would be lost on a hard Brexit,” he said. “The company could re-centre its engineering in Germany and get approval out of there through its membership of EASA.”

Mahf Khan, deputy staff convenor at Rolls-Royce, agreed with Mr Hemmings’ assessment that a hard Brexit would result in thousands of job losses at the Sinfin campus.

“Will it ever decline into nothing? That’s probably unlikely with Derby being the home of Rolls-Royce,” Mr Khan said. “But we know it’s declined over the past 15-20 years from circa. 24,000 employees to around 12,000 now in Derby. We could easily see half of that go, maybe a bit less.”

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Mr Hemmings agreed: “I would say around 5,000 jobs could go in Derby. That wouldn’t be overnight. The jobs would just drift away. From 2022 onwards, we would start to see a gradual disappearing of jobs – and that would also have an impact on jobs in the supply chain.”

On Tuesday, Unite officials met with MPs in Westminster to voice their concerns. Meetings with MEPs also took place in Brussels today.

Rolls-Royce chief executive Warren East has also previously warned against a hard Brexit. Speaking in June, he called for "as little change as possible" after Brexit, adding: "The further we get from the status quo, the harder it's going to be."